In recent years, the photocatalytic activity exhibited by titanium oxide has been applied to various environmental cleaning techniques such as deodorization, prevention of bacterial growth, and antifouling. The band gap of anatase-form titanium oxide which is generally used as a photocatalyst is approximately 3.2 eV. With this titanium oxide, photocatalytic reactions proceed when it receives ultraviolet light having a wavelength shorter than approximately 380 nm. Accordingly, in order for it to exhibit its photocatalytic activity, it is necessary for it to be irradiated by ultraviolet light. Therefore, it has limitations with respect to the environment of installation and use.
If visible light which is present in large amounts in sunlight and indoor light can be utilized as an energy source for a photocatalyst, the reaction activity of the photocatalyst can be strengthened, and it can be utilized in various places. Accordingly, development of materials which can exhibit photocatalytic activity in response to irradiation with visible light is being promoted.
Photocatalysts which have activity in response to visible light (visible light photocatalytic activity) include the following types:
(1) A nitrogen type which contains nitrogen in titanium oxide (e.g., Chem. Phys. Lett. 123 (1986), 126-128; Journal of Japan Chemical Society, 1986 (8), p. 1084; and WO 01/010552);
(2) An oxygen defect type in which oxygen defects are introduced into titanium oxide (e.g., JP 2001-205103 A1); and
(3) A metal doped type which titanium oxide contains another metal (ion) therein or a metal oxide combined therewith.
As described below, many examples of a photocatalyst of the metal doped type (3) have been proposed.
For example, JP 09-262482 A1 discloses titanium oxide which contains vanadium or chromium added by ion implantation.
In Chemical Commun. 2001, 2718-2719, titanium oxide having activity to visible light and containing a transition metal such as V, Cr, Nb, or Mo is reported.
JP 2004-43282 A1 discloses a method for preparing a titanium oxide photocatalyst by mixing various metal compounds with titanium oxide.
JP 2004-275999 A1 discloses titanium oxide which contains a compound of a metal selected from Si, Ti, V, Sn, Sb, W, Nb, Bi, P, Mo, Cs, Ge, As, Ce and the like.
J. Mat. Sci. Lett. 21, 2002, 1655-1656 provides a report concerning an increase in catalytic activity of a titanium oxide-based photocatalyst and a shift toward a longer side in the wavelength of the absorption spectrum of the catalyst caused by incorporation of bismuth.